The importance of this middah is evident from the many instances in which the Torah mentions it. We see that Moshe Rabbeinu would not hit the river by מַכַּת דָּם and צְפַרְדֵּעַ because it protected him as a newborn when Pharoah decreed that the Jewish baby boys be thrown into the Nile. He would not hit the ground by מַכַּת כִּנִּים because he had benefited from it when he killed a Mitzri in retaliation for his hitting a Yid, and then buried the corpse underground to hide the deed. This was the degree of הַכָּרַת הַטּוֹב Moshe had for inanimate objects. This reinforces how fundamental this middah is, and how much more so we must act in this manner in relation to other human beings.
Appreciation even for the Inanimate
The Gemara (ב"ק צב:) tells us 'do not throw a stone into a well from which you have drawn water. Rashi explains that this means that one should not disregard or disparage an object that has previously benefited him. If one has benefited from a place or thing, he must remain grateful to it and not damage it, even though it is inanimate.
The Gemara bases this on a possuk in last weeks parshah לֹא תְתַעֵב מִצְרִי (דברים כג ח) you should not completely distance the Mitzri. The Torah tells us that a male from the nation of Amon or Moav may never marry into the Jewish nation. However, a Mitzri, after converting his third generation will be permitted to marry a Yid, כִּי גֵר הָיִיתָ בְאַרְצוֹ. Rashi explains that this is because they hosted us in their land. Even though they enslaved us brutally, still we must show some measure of appreciation because we lived in their land! Yaakov and his children came to live in Mitzrayim, and even though they later held us against our will, we must still show appreciation for their earlier kindness. This is the extent to which we must never forget even the smallest measure of kindness shown to us!
The ר"י מיגש relates a story that happened with his Rebbe the ר"יף, which personifies the teaching of this Gemara. The ר"יף was once very ill, and a rich man who owned a healing bathhouse allowed him to use these baths to help him recuperate from his illness. The bathhouse did indeed aid in his recovery, and eventually, he recovered completely. After some years the wealthy man lost all his money and had to sell all his possessions in order to pay his heavy debts. The ר"יף was approached as a halachic authority was needed to help facilitate the sale. The ר"יף said that he is unable to aid in the sale of the bathhouse, as he had greatly benefited from it. He did not want to disadvantage a place which he had used, and he was afraid that perhaps the sale was not beneficial for the bathhouse!
The ר"י מיגש expresses a קַל וָחֹמֶר, a fortiori that we must learn from this story. If his Rebbe showed so much appreciation for an inanimate bathhouse, how much more so must we show appreciation to a human being who has granted us a favor.
The Vilna Gaon quotes the רמב"ם who wrote in מוֹרֶה נְבוֹכִים that if a person learned even one thing from a sefer, he should not scoff at the rest of the sefer. He should show gratefulness for the benefit he derived from the sefer, and therefore he should not demean it. (ענף יוסף, עין יעקב שם)
A Yiddishe Middah
The של"ה הקדוש quotes an interesting תַּרְגּוּם יוֹנָתָן. Targum Yonoson says that when the Mitzrim could no longer afford to buy food during the famine in Mitzrayim, Yosef told them to sell their land to the government and thus he would provide them food as a form of payment. Their land was repossessed by the government. However, אֲדְמַת הַכֹּהֲנִים לֹא קָנָה (בראשית מז כב) Yosef did not acquire the land of the 'priests of Mitzrayim. He allowed the 'priests to keep their land even if they did not have money to buy food. This was in appreciation for an earlier act of kindness they had shown him.
When the אֵשֶׁת פּוֹטִיפַר made a libel on Yosef, he was taken to court before the priests of Mitzrayim. They realized his innocence and therefore did not give him capital punishment. Perhaps they could not fully negate the accusation of this 'noble women, so they sentenced him to prison. However, the true punishment for this crime would have been a death sentence, and they spared Yosef from that. Many years later, in recognition and appreciation for their kindness, Yosef did not tax their land in their time of need. Our Fathers and forebearers showed us the importance of הַכָּרַת הַטּוֹב.